A linear actuator imparts a motion of an object along a linear path, e.g., back and forth, by means of applying a force to the object directed along the path traversed by the object. In an electromagnetic actuator, the force is generated through the interaction between a magnetic field and actively controlled electric currents flowing in conductors. The conductors are mounted at a piece of ferromagnetic material that is moveable with respect to the magnets generating the magnetic field.
The object is attached, or otherwise mechanically coupled, to either the piece of ferromagnetic material or the magnets. The ferromagnetic material serves to concentrate the flux of the magnetic field, thereby increasing the magnetic field strength at the locations of the conductors. As known, an electric charge moving in a magnetic field experiences a force, referred to as the Lorentz force. In the actuator, the Lorentz force acts on the conductors. The action-reaction principle of Newton's third law stipulates that a reaction force then acts on the magnets. The magnets and conductors in the actuator can move relative to one another, as a result of which the acting forces cause a relative motion. The magnetic field as experienced by the conductors may typically depend on the relative position of the conductors with respect to the magnets. The direction of the magnetic field vector may even change its polarity. Accordingly, the currents may need to be actively controlled in dependence on the position of the conductors with respect to the magnets.
Japanese patent application publication 2005106242 discloses an electromagnetic actuator with a damping force that is the combination of a first force, whose magnitude and direction depend on controlled currents in coils, and a second force, which is caused by passively inducing eddy currents. The damping force has a magnitude and a direction that depend on a relative velocity of two components of the known actuator. In case of an electric failure, the second force is still present and serves as a back-up. The actuator is used in a rail car and is located between the cart and the passenger compartment.
More specifically, the known actuator comprises a cylindrical shaft carrying on its outer surface a plurality of magnets. The shaft is moveable back and forth in a stator core. The stator core comprises a stack of ring-shaped, uniform units of a ferromagnetic material enclosing the shaft. Each unit has a profiled cross-section leaving a ring-shaped recess between two adjacent units of the stack in the inner wall of the stator core facing the shaft. The recess accommodates a coil. The magnets are ring-shaped magnets, positioned at suitable intervals around the shaft or, alternatively, tile-shaped magnets arranged in rings or in a spiral. The spatial distribution of the magnets along the shaft reflects the regular distance between neighbouring coils in the stator core. A sensor provides a sensor signal indicative of the position of the shaft relative to the stator core. From this sensor signal can be derived the relative velocity and relative acceleration of the shaft with respect to the stator core. This information is then used to actively provide damping via drive currents supplied to the coils. The passive damping results from eddy currents generated in the stator core's material by the magnets on the moving shaft. Damping force can be adjusted by choosing a ferromagnetic material for the stator core with a suitable volume resistivity. Furthermore, a radial slit is provided in a unit of the stator core that acts to restrict the eddy currents in order to further adjust the damping force.
An analysis of another known type of electromagnetic actuator is described in, e.g., “Tubular Modular Permanent-Magnet Machines Equipped With Quasi-Halbach Magnetized Magnets—Part I: Magnetic Field Distribution, EMF and Thrust Force”, Jiabin Wang and David Howe, IEEE Transactions on Magnetics, Vol. 41, No. 9, September 2005, pp. 2470-2478; and “Tubular Modular Permanent-Magnet Machines Equipped With Quasi-Halbach Magnetized Magnets—Part II: Armature Reaction and Design Optimization”, Jiabin Wang and David Howe, IEEE Transactions on Magnetics, Vol. 41, No. 9, September 2005, pp 2479-2489.
This known type of actuator has an outer tube with a linear array of ring-shaped magnets and an inner tube of a ferromagnetic material accommodating a linear array of coils. The outer tube surrounds the inner tube for at least part of its length, and the tubes are positioned coaxially. Both tubes have uniform cross sections. The coils are oriented so as to be substantially coaxial with the tubes. A gap, e.g., an air gap, spatially separates the inner and outer tubes from one another. The linear array of magnets is configured so as to create a magnetic field in the gap that is predominantly perpendicular to the common axis of the tubes. The polarity of the direction of the magnetic field depends on the location relative to the array of magnets. The array of magnets is implemented by means of, e.g., a cylindrical Halbach array. The inner tube is kept stationary and the outer tube is axially moveable with respect to the outer tube. Selectively driving the coils with currents of the proper polarity causes the outer tube to move as a result of a force on the magnetic array that is the reaction to the Lorentz forces on the coils. The spatial distribution of the coils along the inner tube and the spatial distribution of the magnets along the outer tube together determine which coils have to be driven in order to move the outer tube in a predetermined direction. Assume that the magnetic field in the gap has a regular pattern of regions of alternating polarity. Assume further that the coils are uniform and have a regular spatial distribution along the inner tube. Then, in order to control the outer tube's movement, different sub-sets of coils can be driven with different currents. Such a driving system is referred to as a multi-phase system. The currents are uniform per sub-set but have magnitudes and polarities that depend on the relative axial positions of the tubes. As the force applied to a driven coil is the Lorentz force, the magnitude of the force is proportional to the magnitude of the current in the coil. The coils are uniform and regularly distributed, as a result of which the combined force on the uniformly driven sub-set of coils is still proportional to the magnitude of the current. The inner tube is held stationary and, accordingly, the reaction force on the array of magnets has the same magnitude but has a direction opposite to the Lorentz force.
The current flow through the coils generates an additional magnetic field on its own. The total magnetic field is then the combination of the magnetic field of the magnets and this additional magnetic field. The magnitude of the additional magnetic field is, in general, much lower than the magnitude of the magnetic field of the magnets. However, for extreme current values, the magnitude of the additional magnetic field may become substantial. The magnitude of the magnetic field in the ferromagnetic material significantly increases until saturation of the ferromagnetic material is reached. As a result of the saturation, an increase of the driving current will then lead to an increase in the magnitude of the force that is substantially lower than proportional.
As pointed out in Japanese patent application publication 2005106242, a magnetic actuator can be used as an element in an actively controlled damping system as part of the suspension of a vehicle. In an active suspension system, the vehicle's conditions are electronically monitored. Information from wheel sensors (indicative of suspension extension), steering, and acceleration sensors is used to calculate the optimal stiffness. Control signals are then generated that determine the currents in the magnetic actuator, as a result of which the rate of movement, i.e., the compression-rebound rate, in the vehicle suspension is controlled, practically in real-time. Thus, the motion of the vehicle is directly controlled and its riding characteristics are improved. A problem with a suspension system that relies on active control only, is that no adequate damping may be provided if the system malfunctions, e.g., as a result of an electric failure in the control circuitry or a loosened connector or broken wire. The presence of an additional passive damping mechanism is therefore preferred.